1. Field of the Invention
The invention relates to copolymers of ethylenically unsaturated monomers and of ethylenically functionalized nanoparticles in the form of their aqueous dispersions or water-redispersible powders, and to their preparation and use.
2. Description of the Related Art
Polymer dispersions which comprise particles having dimensions in the nanometer range, e.g. particles measuring less than 100 nm in at least one dimension, have a host of innovative properties which are superior to those of composites containing particles which are not so fine, in the micrometer range, for instance. These properties include, for example, light scattering, adsorbency and absorbency, antibacterial properties, or superior scratch resistance and tensile strength. These “nanoeffects” correlate directly to the size of the particles and are lost if the particles exceed certain dimensions.
Furthermore, the desired effects are only particularly pronounced when success is achieved in distributing the particles as homogeneously as possible in the polymer matrix and, if possible, attaching them chemically to it, in order to prevent leaching or agglomeration phenomena and hence a loss of these special properties.
One way of chemically attaching nanoscale metal oxides to polymeric matrices is described for example in DE 10212121 A1 for nano-zinc oxide polymer dispersions. The zinc oxide particles are dispersed in a halogen-containing medium, the dispersion is introduced into an aqueous solution of hydroxyl-containing inorganic polymers, such as of hydrolysed polyalkyl(alkoxy)siloxanes, and then the halogen-containing constituents are removed by distillation. Chemical attachment to the polymer thus takes place via the formation of a Zn—O—Si—O—C bridge and is therefore very unstable with respect to acidic or alkaline cleavage.
Where the particles are silicone resins it is known that they can be used for chemical modification of organic polymers or as binders of coatings in order to increase the resistance of the coatings, for example, to weathering effects, chemical attack and thermal loading. Commercially available products are, for example, silicone polyesters, hybrid systems of silicone resins and organic polymers of the kind used to produce coil coatings. These products are preferably prepared by chemical reaction and bond formation between the silicone resin and the organic polymer. Chemical attachment of the silicone resins to the organic polymer takes place in general by the formation of an Si—O—C bridge between the two, typically in a solvent operation. For aqueous media the literature includes a variety of products comprising combinations of organic polymers with silicone resins or resin-like oligomeric silicone structures, and processes for their preparation:
EP 1256611 A2 describes an aqueous dispersion obtained from a mixture and emulsion of non-free-radically polymerizable alkoxysilanes or their hydrolysis and condensation products with free-radically polymerizable monomers. The silanes or products derived therefrom are hydrolysed and condensed, while the organic monomers are free-radically polymerized. The silanes used in this case are alkyl- or arylalkoxysilanes, for which there can be up to three alkoxy groups attached to silicon. It is possible therefrom, by means of hydrolysis and condensation, to access resins or resin-like oligomers, inter alia.
EP 1197502 A2 teaches the preparation of an aqueous resin emulsion by free-radical polymerization of ethylenically unsaturated monomers in the presence of hydrolysable and condensable mono-, di- or trialkoxyalkyl- or -aryl-silanes which are not free-radically polymerizable.
EP 943634 A1 describes aqueous latices intended for use as coating materials and prepared by copolymerizing ethylenically unsaturated monomers in the presence of a silicone resin containing silanol groups. In this case interpenetrating networks (IPNs) are formed between the polymer chains and the polysiloxane chains.
The silicone resin emulsion polymers obtainable by the stated processes, and also the otherwise well-known physical mixtures of silicone resin emulsions and organic polymer dispersions, for use for example in the field of silicone resin masonry paints, are notable for the fact that the silicone resin and the organic polymer are present, exclusively or predominantly, in the form of physical blends. Chemical bonds between the silicone fraction and the organic fraction form on more of a chance basis, and are Si—O—C bonds, which are susceptible to hydrolysis. The Si—O—C bond is always in competition with the formation of Si—O—Si bridges as a result of condensation of the silanol groups with one another.
The condensation reactions of the silane units or their hydrolysed and partially-condensed oligomers under the hydrolytic conditions of emulsion polymerization cannot be adequately controlled. It is known that alkoxysilanes having short, oxygen-attached alkyl radicals, in particular, have a pronounced tendency, under hydrolytic conditions, to continue condensing until they form solid particles. These particles tend to form precipitates and domains, and hence tend towards separation. The greater the number of alkoxy groups attached to the silicon, the more pronounced this tendency. In a coating material application, this may have an adverse effect in the form of pinholes being formed. Separation may detract from the storage stability and functional capacity of the products.
A more defined attachment of the silicone unit to the organic polymer, via the formation of C—C bonds, may be accomplished by copolymerizing double bond-functionalized silicones with organic monomers. For example, EP 1308468 A1 describes hydrophobically modified copolymers obtained by copolymerizing linear silicones having up to two polymerizable groups in emulsion with organic monomers. A similar approach is taken by EP 352339 A1, in which vinyl-terminated, linear polydimethylsiloxanes are copolymerized with (meth)acrylate monomers. EP 771826 A2 describes the emulsion polymerization of (meth)acrylic esters and vinylaromatics, crosslinking being effected by addition of difunctional silicones containing acrylic or vinyl groups. EP 635 526 A1 describes functional graft polymers which are based on organopolysiloxanes and are obtained by grafting ethylenically unsaturated monomers onto polyorganosiloxanes which contain hydrogen or functional groups, and also ethylenically unsaturated groups.
The preparation of organocopolymer dispersions containing particles is subject matter of EP 1216262 B1 and EP 1235869 B1, where an aqueous dispersion of inorganic particulate solids and organic polymer is prepared using inorganic particulate solids which are characterized by a defined degree of dispersion and a defined electrophoretic mobility and in whose presence ethylenically unsaturated monomers are polymerized. EP 505230 A1 describes the encapsulation of silica particles with organic polymer, the silica particles first being functionalized with ethylenically unsaturated alkoxysilane compounds and then ethylenically unsaturated monomers being polymerized in aqueous dispersion in the presence of the functionalized particles.